Commutating circuit

ABSTRACT

This disclosure relates to a commutating circuit for alternately fully energizing a first load and a second load for use in highway crossing warning applications. Associatively connected to the first and second loads are first and second gates through first and second windings of a commutating transformer, respectively. A source of gating alternately triggers the first and second gates to fully energize the load associated therewith. The commutating transformer alternately disables the first and second gates; thereby alternately substantially deenergizing the first and second loads.

United States Patent Darrow [54] COMMUTATING CIRCUIT [72] Inventor: I John O. G. Darrow, Murrysville, Pa.

[73] Assignee: Westinghouse Air Brake Company,

Swissvale, Pa.

22 Filed: Feb. 16,1971

21 App1.No.: 115,315

[451 Sept. 26, 1972 Weingarden ..3 15/ l 32 Bedford ..307/252 M Primary Examiner-Rudolph V. Rolinec Assistant Examiner-David M. Carter Attorney-H. A. Williamson [57] ABSTRACT This disclosure relates to a commutating circuit for alternately fully energizing a first load and a second load for use in highway crossing warning applications. Associatively connected to the first and second loads are first and second gates through first and second windings of a commutating transformer, respectively. A source of gating alternately triggers the first and second gates to fully energize the load associated [56] References Cited therewlth. The commutatmg transformer alternately UNITED STATES PATENTS disables the first and second gates; thereby alternately 3,283,206 11/1966 Un ..307/252 M substantially energizing the first and wads 3,502,901 3/1970 Tumozawa ..307/286 18 Claims, 1 Drawing Figure COMMUTATING CIRCUIT My invention relates to a commutating circuit for alternately energizing first and second loads, and more particularly to a flashing circuit wherein the first and second loads are incandescent lamps for use in highway crossing warning applications.

The dangers attendant the intersection of highways, streets and the like with railroad roadways are almost universally known. To provide warnings for approaching pedestrians and automotive vehicles as well as other vehicular traffic various types of alarm circuits for indicating an approaching railway vehicle are used. These circuits provide either an audible or a visible warning to approaching pedestrians or vehicles. The latter type of warning circuits customarily employ a lighting arrangement whereby warning lights are turned on continuously or are caused to flash while a railway vehicle moves within the highway crossing area. It is particularly the flashing type circuitry with which the instant invention concerns itself. A number of types of flashing circuits for highway crossing warning application are substantially electromechanical in nature. That is, the lamps incorporated in these circuits are caused to flash on and off via the use of relays and various contact arrangements. Accordingly, frequent maintenance and adjustment is necessitated and entails long term expense. The flashing of lights in other prior types of flashing circuits is achieved by commutation through the use of a commutating capacitor, undesirable because of its required bulkiness and expensive cost.

It is therefore an object of my invention to provide a novel flashing circuit arrangement which is entirely solid state in nature.

A further object of my invention is to provide an improved flashing circuit arrangement which eliminates the employment of a commutating capacitor, and wherein commutation is achieved through the use of a commutating transformer.

Another object of my invention is to provide a new flashing circuit arrangement wherein the impedance conditions of the lamps to be flashed are instrumental in enhancing commutation.

Yet another object of my invention is to provide a new and improved flashing circuit arrangement which is simple in maintenance and construction, economical in cost, and efficient and reliable in operation.

In the attainment of the foregoing objects, a flashing circuit has been invented which comprises a first and a second incandescent lamp which are serially connected; a power source connected across the serial connection of the incandescent lamps; a commutating transformer having a first and a second winding; a first and a second silicon controlled rectifier, each having a gate electrode, an anode electrode and a cathode electrode; a source of gating; and a coupling transformer having a primary winding connected across the source of gating, and first and second secondary windings.

One end of the first secondary winding of the coupling transformer is connected to the gate electrode of the first silicon controlled rectifier, while the other end is connected to the cathode electrode of the first silicon controlled rectifier.

One end of the second secondary winding of the coupling transformer is connected to the gate electrode of the second silicon controlled rectifier, while the other end is connected to the cathode electrode of the second silicon controlled rectifier.

The source of gating alternately triggers the first and the second silicon controlled rectifiers through the coupling transformer, the primary winding of which, as previously mentioned, is connected across the source of gating.

The cathode electrode of the first silicon controlled rectifier is also connected to the junction of the first and the second lamps through the first winding of the commutating transformer for energizing the first lamp. The anode electrode of the first silicon controlled rectifier is connected to the second lamp.

The anode electrode of the second silicon controlled rectifier is connected to the junction of the first and the second lamps through the second winding of the commutating transformer for energizing the second lamp. The cathode electrode of the second silicon controlled rectifier is also connected to the first lamp.

The alternate triggering of the first and the second silicon controlled rectifiers causes the respective alternate full energization of the first and the second lamps.

The first and the second lamps have a relatively high impedance condition when fully energized, and a relatively low impedance condition when substantially deenergized. A low impedance condition in one of the lamps and a high impedance condition in the other lamp causes the voltage across the one winding of the commutating transformer through which the lamp of low impedance is to be energized to be of a particular polarity whenever the silicon controlled rectifier associated with the one winding of the commutating transformer is triggered. A voltage of a polarity to cause disabling of the other silicon controlled rectifier is thereby induced in the other winding of the commutating transformer through which the lamp of high impedance has been energized.

A diode and a fuse are serially interconnected between the first and second lamp. The diode prevents both silicon controlled rectifiers from shorting the power source should one of the lamps become opencircuited. The fuse will blow if the diode becomes short-circuited and both silicon controlled rectifiers are triggered on causing both of the lamps to light.

Other objects and advantages of the present invention will become apparent from the ensuing description of the illustrative embodiment thereof, in the course of which reference is had to the accompanying drawing which illustrates in schematic form a preferred embodiment of the flashing circuit arrangement of the present invention.

Reference is now made to the drawing which illustrates my invention. Shown in the drawing is a power source, or battery Ba, selectively connected across the serial connection of an incandescent lamp B2, a diode D1, a fuse F1 and another incandescent lamp Bl via a switch S. Incandescent lamps B1 and B2 are chosen such that the internal cold resistance of each is relatively low, while the hot resistance which is reached after a thermal time constant is relatively high. Typically the cold resistance of lamps B1 and B2 is approximately one tenth the hot resistance. Also shown is a source of gating l, which is preferably a conventional type pulse source producing pulses of alternating polarity. As depicted, the source of gating l is transformer coupled via coupling transformer T1 to a silicon controlled rectifier SCRl having a gate electrode 11, an anode electrode 12, and a cathode electrode 14, and also to a silicon controlled rectifier SCR2 having a gate electrode 18, an anode electrode 19 and a cathode electrode 21. The coupling transformer T1 has a primary winding 2 connected across the source of gating, and two secondary windings 5 and 6. The windings 5 and 6 are arranged such that voltages of alternating polarity at winding 2 will induce voltages of alternating polarity in windings 5 and 6, respectively. In particular, the source of gating l iscoupled to the siliconcontrolled rectifier SCRl through primary winding 2 of coupling transformer T1, and secondary winding 5 of coupling transformer T1. As shown, one end of the secondary winding 5 of coupling transformer T1 is connected to the gate electrode ll of silicon controlled rectifier SCRl, while the other end of secondary winding 5 of coupling transformer T1 is connected to the cathode electrode 14 of silicon controlled rectifier SCRL Further, one end of the secondary winding 6 of coupling transformer T1 is connected to the gate electrode 18 of silicon controlled rectifier SCR2, while the other end of the secondary winding 6 of coupling transformer T1 is connected to the cathode electrode 21 of silicon controlled rectifier SCR2.

. The cathode electrode 14 of silicon controlled rectifier SCRl is also connected to the junction of fuse F1 and lamp Bl through one winding 3 of a commutating transformer T2, and the anode electrode 12 of silicon controlled rectifier SCRl is connected to the lamp B2 via a common lead 25.

The anode electrode 19 of silicon controlled rectifier SCR2 is connected to the junction of the anode electrode of diode Dl and lamp Bl through another winding 4' of the commutating transformer T2, and the cathode electrode 21 of silicon controlled rectifier SCR2 is also connected to the lamp B1 via a common lead 26.

Turning nowto the operation of the flashing circuit arrangement of the drawing, it will be assumed that an external signal circuitry not shown will cause the closing of the switch S and, in turn, the connection of the battery Ba across lamps B1 and B2. For example, switch S may be closed via a signal from track circuitry at a railroad highway crossing upon the detection of an approaching vehicle within the crossing area. Simultaneously, the source of gating 1 will be turned on through circuitry not shown. Since both lamps B1 and B2 were initially off, the internal resistance presented by each when switch S is closed is the previously mentioned relatively low cold resistance. Now, assuming that upon the turning on of the source of gating l, a pulse from the source of gating l is of a polarity to cause'the inducing of a voltage in secondary winding 5 of coupling transformer T1 by primary winding 2 of a polarity such that a positive potential appears at gate electrode 11 of silicon controlled rectifier SCRl, then the silicon controlled rectifier SCRl will be gated on through its gate electrode 11 and will begin conducting. Upon the conduction of silicon controlled rectifier SCRl, a path will be completed from the positive terminal of battery Ba over switch S, lead 25, anode electrode 12 of silicon controlled rectifier SCRl, cathode electrode 14 of silicon controlled rectifier SCRl, winding 3 of commutating transformer T2, lamp B1, to the negative terminal of battery Ba, and a positive voltage will momentarily appear across winding 3 of commutating transformer T2 because the bottom end of winding 3 of commutating transformer T2 is held near zero voltage due to the relatively low cold resistance of lamp Bl. This momentary positive voltage across winding 3 of commutating transformer T2 will induce a voltage in winding 4 of commutating transformer T2, the bottom end of that winding being negative with respect to the top end, thereby reverse biasing silicon controlled rectifier SCR2 and maintaining it turned off. Since a dc. signal is being delivered from the battery Ba, the voltage across winding 3 of commutating transformer T2, which winding has a finite low inductance, will vanish. After a very short time determined by its thermal time constant, lamp Bl will light to full brilliance, while lamp B2 will be shorted out via the path through anode electrode 12 of silicon controlled rectifier SCRl, cathode electrode 14 of silicon controlled rectifier SCRl, and the winding 3 of commutating transformer T2. Similarly, since winding 4 of commutating transformer T2 has a finite small inductance, the voltage drop across it will vanish and this portion of the switching cycle is complete.

Now, let us assume that a pulse of opposite polarity from the source of gating 1 causes a voltage of opposite polarity to appear across primary winding 2 of coupling transformer T1. Accordingly, a voltage will be induced in the secondary winding 6 of coupling transformer T1 of a polarity such that a positive potential appears at gate electrode 18 of silicon controlled rectifier SCR2, and silicon controlled rectifier SCR2 will be gated on through its gate electrode 18 and will begin conducting. Upon the conduction of silicon controlled rectifier SCR2 a path will be completed from the positive terminal of battery Ba over switch S, lamp B2, winding 4 of commutating transformer T2, anode electrode 19 of silicon controlled rectifier SCR2, cathode electrode 21 of silicon controlled rectifier SCR2, lead 26, to the negative terminal of battery Ba, and a positive voltage will now momentarily appear across the winding 4 of commutating transformer T2 because the top end of winding 4 of commutating transformer T2 is held near the positive battery potential due to the relatively low cold resistance of lamp B2. This momentary positive voltage across winding 4 of commutating transformer T2 will now induce a voltage in winding 3 of commutating transformer T2, the top end of that winding being positive with respect to the bottom end, thereby reverse biasing silicon controlled rectifier SCRl and causing it to turn off and no longer conduct. Since a dc. signal is being delivered from the battery Ba, the voltage across winding 4 of commutating transformer T2 will vanish. After a very short time determined by its thermal time constant, lamp B2 will light to full brilliance, while lamp Bl will be shorted out via the path through winding 4 of commutating transformer T2, anode electrode 19 of silicon controlled rectifier SCR2,

and cathode electrode 21 of silicon controlled rectifier SCR2. Similarly, since winding 3 of commutating transformer T2 has a finite, small inductance, the voltage drop across it will vanish, and the switching cycle is complete. Diode D1 prevents both silicon controlled rectifiers SCRI and SCR2 from shorting the battery Ba should one of the lamps B1 or B2 become open-circuited. lf diode D1 becomes short-circuited and both silicon controlled rectifiers SCRl and SCR2 turn on," fuse F1 will blow and both lamps B1 and B2 will light.

While my invention has been described with regard to a flashing or commutating circuit arrangement for highway crossing warning applications, it will be understood that the invention may have utility in other systems and unrelated areas remote from highway crossing applications.

It will be appreciated, therefore, that the foregoing description of my invention is only illustrative and it is not intended that the invention be limited thereto. Thus, sundry variations, alterations and modifications may be made by those skilled in the art without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. A commutating circuit for alternately fully energizing a first load and a second load and comprising said first load and said second load, a commutating transformer having a first and a second winding, a first and a second gate means respectively associatively connected to said first and said second loads through said first winding and said second winding of said commutating transformer, and a source of gating coupled to said first gate means and said second gate means for alternately triggering said first gate means and said second gate means to fully energize the load associated therewith, said commutating transformer alternately disabling said first gate means and said second gate means, thereby alternately substantially deenergizing said first load and said second load.

2. A circuit as defined in claim 1 wherein said first and said second loads are serially connected and a power source is connected across said serial connection of said loads.

3. A circuit as defined in claim 1 wherein said first and second windings of said commutating transformer alternately act as primary windings in accordance with the alternate triggering of said first and said second gate means.

4. A circuit as defined in claim 1 wherein said first and said second loads have a relatively low impedance condition when substantially deenergized and a relatively high impedance condition when fully energized, a low impedance condition in one of said loads and a high impedance condition in the other of said loads causing the voltage across the one winding of said commutating transformer associated with the load of low impedance to be of a particular polarity whenever the gate means associated with said winding is triggered, thereby inducing a voltage in the other winding of said commutating transformer associated with the load of high impedance of a polarity to cause disabling of the other gate means.

5. A circuit as defined in claim 1 wherein said first gate means is a silicon controlled rectifier having a gate electrode, an anode electrode and a cathode electrode, said gate electrode connected to said source of gating, said anode electrode connected to said second load positive potential and said cathode electrode connected to said first load through said first winding of said commutating transformer.

6. A circuit as defined in claim 1 wherein said second gate means is a silicon controlled rectifier having a gate electrode, an anode electrode and a cathode electrode, said gate electrode connected to said source of gating, said anode electrode connected to said second load through said second winding of said commutating transformer and said cathode electrode connected to said first load through a preselected reference potential.

7. A circuit as defined in claim 1 wherein said first and said second gate means are transformer coupled to said source of gating.

8. A circuit as defined in claim 2 wherein a diode and a fuse are interconnected between said first and said second loads.

9. A flashing circuit comprising a first and a second lamp, a commutating transformer having a first and a second winding, a first and a second gate means respectively associatively connected to said first and said second lamps through said first winding and said second winding of said commutating transformer, and a source of gating for alternately triggering said first gate means and said second gate means to fully energize the lamp associated therewith, said commutating transformer alternately disabling said first gate means and said second gate means, thereby alternately substantially deenergizing said first lamp and said second lamp.

10. A circuit as defined in claim 9 wherein said first and saidsecond lamps are incandescent lamps.

11. A circuit as defined in claim 9 wherein said first and said second lamps are serially connected and a power source is connected across said serial connection of said lamps.

12. A circuit as defined in claim 9 wherein said first and said second windings of said commutating transformer alternately act as, primary windings in accordance with the alternate triggering of said first and said secondgate means.

13. A circuit as defined in claim 9 wherein said first and said second lamps have a relatively low impedance condition when substantially deenergized and a relatively high impedance condition when fully energized, a low impedance condition in one of said lamps and a high impedance condition in the other of said lamps causing the voltage across the one winding of said commutating transformer associated with the lamp of low impedance to be of a particular polarity whenever the gate means associated with said one winding is triggered, thereby inducing a voltage in the other winding of said commutating transformer associated with the lamp of high impedance of a polarity to cause disabling of the other gate means.

14. A circuit as defined in claim 9 wherein said first gate means is a silicon controlled rectifier having a gate electrode, an anode electrode and a cathode electrode, said gate electrode connected to said source of gating, said anode electrode connected to said second lamp and said cathode electrode connected to said first lamp through said first winding of said commutating transformer.

15. A circuit as defined in claim 9 wherein said second gate means is a silicon controlled rectifier having a gate electrode, an anode electrode and a cathode electrode, said gate electrode connected to said source of gating, said anode electrode connected to said second lamp through said second winding of said commutating transformer and said cathode electrode connected to said first lamp through a preselected reference potential.

16. A circuit as defined in claim 9 wherein said first and said second gate means are transformer coupled to said source of gating.

17. A circuit as defined in claim 11 wherein a diode and a fuse are interconnected between said first and said second lamps.

18. A flashing circuit'comprising,

a. a first and a second incandescent lamp which are serially connected, a diode and a fuse interconnected between said lamps,

b. a power source connected across said serial connection of said first and said second lamps,

c. a commutating transformer having a first and a Y second winding,

d. a first and a second silicon controlled rectifier each having a gate electrode, an anode electrode and a cathode electrode, 1

e. a source of gating, and

f. a coupling transformer having a primary winding connected across said source of gating and first and second secondary windings, one end of said first secondary winding of said coupling transformer connected to .said gate electrode of said first silicon controlled rectifier, the other end of said first secondary winding of said coupling transformer connected to said cathode electrode of said first silicon controlled rectifier, one end of said second secondary winding of said coupling transformer connected to said gate electrode of said second silicon controlled rectifier, the other end of said second secondary winding of said coupling transfonner connected to said cathode electrode of said second silicon controlled rectifier, said source of gating alternately triggering said first and said second silicon controlled rectifiers through said coupling transformer, the primary winding of which is connected across said source of gating,

said cathode electrode of said first silicon controlled rectifier also connected to the junction of said first and said second lamps through said first winding of said commutating transfonner for energizing said first lamp, said anode electrode of said first silicon controlled rectifier connected to said second lamp,

said anode electrode of said second silicon controlled rectifier connected to the junction of said first and said second lamps through said second winding of said commutating transformer for energizing said second lamp, said cathode electrode of said second silicon controlled rectifier also connected to said first lamp,

the alternate triggering of said first and said second silicon controlled rectifiers causing the respective alternate full energization of said first and said second lamps,

said first and said second lamps having a relatively high impedance condition when fully energized and a relatively low impedance condition when substantially deenergized, a low impedance condition ino eof said lam s d ahi him dance condition in the other 0 s iil lamgs cau ing the voltage across the one winding of said commutating transformer through which said lamp of low impedance is energized to be of a particular polarity whenever the silicon controlled rectifier associated with the one winding of said commuta'ting transformer is triggered, thereby inducing a voltage in the other winding of said commutating transformer, through which said lamp of high impedance has been energized, of a'polarity to cause disabling of the other silicon controlled rectifier. 

1. A commutating circuit for alternately fully energizing a first load and a second load and comprising said first load and sAid second load, a commutating transformer having a first and a second winding, a first and a second gate means respectively associatively connected to said first and said second loads through said first winding and said second winding of said commutating transformer, and a source of gating coupled to said first gate means and said second gate means for alternately triggering said first gate means and said second gate means to fully energize the load associated therewith, said commutating transformer alternately disabling said first gate means and said second gate means, thereby alternately substantially deenergizing said first load and said second load.
 2. A circuit as defined in claim 1 wherein said first and said second loads are serially connected and a power source is connected across said serial connection of said loads.
 3. A circuit as defined in claim 1 wherein said first and second windings of said commutating transformer alternately act as primary windings in accordance with the alternate triggering of said first and said second gate means.
 4. A circuit as defined in claim 1 wherein said first and said second loads have a relatively low impedance condition when substantially deenergized and a relatively high impedance condition when fully energized, a low impedance condition in one of said loads and a high impedance condition in the other of said loads causing the voltage across the one winding of said commutating transformer associated with the load of low impedance to be of a particular polarity whenever the gate means associated with said winding is triggered, thereby inducing a voltage in the other winding of said commutating transformer associated with the load of high impedance of a polarity to cause disabling of the other gate means.
 5. A circuit as defined in claim 1 wherein said first gate means is a silicon controlled rectifier having a gate electrode, an anode electrode and a cathode electrode, said gate electrode connected to said source of gating, said anode electrode connected to said second load positive potential and said cathode electrode connected to said first load through said first winding of said commutating transformer.
 6. A circuit as defined in claim 1 wherein said second gate means is a silicon controlled rectifier having a gate electrode, an anode electrode and a cathode electrode, said gate electrode connected to said source of gating, said anode electrode connected to said second load through said second winding of said commutating transformer and said cathode electrode connected to said first load through a preselected reference potential.
 7. A circuit as defined in claim 1 wherein said first and said second gate means are transformer coupled to said source of gating.
 8. A circuit as defined in claim 2 wherein a diode and a fuse are interconnected between said first and said second loads.
 9. A flashing circuit comprising a first and a second lamp, a commutating transformer having a first and a second winding, a first and a second gate means respectively associatively connected to said first and said second lamps through said first winding and said second winding of said commutating transformer, and a source of gating for alternately triggering said first gate means and said second gate means to fully energize the lamp associated therewith, said commutating transformer alternately disabling said first gate means and said second gate means, thereby alternately substantially deenergizing said first lamp and said second lamp.
 10. A circuit as defined in claim 9 wherein said first and said second lamps are incandescent lamps.
 11. A circuit as defined in claim 9 wherein said first and said second lamps are serially connected and a power source is connected across said serial connection of said lamps.
 12. A circuit as defined in claim 9 wherein said first and said second windings of said commutating transformer alternately act as primary windings in accordance with the alternate triggering of said first and said secoNd gate means.
 13. A circuit as defined in claim 9 wherein said first and said second lamps have a relatively low impedance condition when substantially deenergized and a relatively high impedance condition when fully energized, a low impedance condition in one of said lamps and a high impedance condition in the other of said lamps causing the voltage across the one winding of said commutating transformer associated with the lamp of low impedance to be of a particular polarity whenever the gate means associated with said one winding is triggered, thereby inducing a voltage in the other winding of said commutating transformer associated with the lamp of high impedance of a polarity to cause disabling of the other gate means.
 14. A circuit as defined in claim 9 wherein said first gate means is a silicon controlled rectifier having a gate electrode, an anode electrode and a cathode electrode, said gate electrode connected to said source of gating, said anode electrode connected to said second lamp and said cathode electrode connected to said first lamp through said first winding of said commutating transformer.
 15. A circuit as defined in claim 9 wherein said second gate means is a silicon controlled rectifier having a gate electrode, an anode electrode and a cathode electrode, said gate electrode connected to said source of gating, said anode electrode connected to said second lamp through said second winding of said commutating transformer and said cathode electrode connected to said first lamp through a preselected reference potential.
 16. A circuit as defined in claim 9 wherein said first and said second gate means are transformer coupled to said source of gating.
 17. A circuit as defined in claim 11 wherein a diode and a fuse are interconnected between said first and said second lamps.
 18. A flashing circuit comprising, a. a first and a second incandescent lamp which are serially connected, a diode and a fuse interconnected between said lamps, b. a power source connected across said serial connection of said first and said second lamps, c. a commutating transformer having a first and a second winding, d. a first and a second silicon controlled rectifier each having a gate electrode, an anode electrode and a cathode electrode, e. a source of gating, and f. a coupling transformer having a primary winding connected across said source of gating and first and second secondary windings, one end of said first secondary winding of said coupling transformer connected to said gate electrode of said first silicon controlled rectifier, the other end of said first secondary winding of said coupling transformer connected to said cathode electrode of said first silicon controlled rectifier, one end of said second secondary winding of said coupling transformer connected to said gate electrode of said second silicon controlled rectifier, the other end of said second secondary winding of said coupling transformer connected to said cathode electrode of said second silicon controlled rectifier, said source of gating alternately triggering said first and said second silicon controlled rectifiers through said coupling transformer, the primary winding of which is connected across said source of gating, said cathode electrode of said first silicon controlled rectifier also connected to the junction of said first and said second lamps through said first winding of said commutating transformer for energizing said first lamp, said anode electrode of said first silicon controlled rectifier connected to said second lamp, said anode electrode of said second silicon controlled rectifier connected to the junction of said first and said second lamps through said second winding of said commutating transformer for energizing said second lamp, said cathode electrode of said second silicon controlled rectifier also connected to said first lamp, the alternate triggering of said first and said second silicon controlled rectifiers causing the reSpective alternate full energization of said first and said second lamps, said first and said second lamps having a relatively high impedance condition when fully energized and a relatively low impedance condition when substantially deenergized, a low impedance condition in one of said lamps and a high impedance condition in the other of said lamps causing the voltage across the one winding of said commutating transformer through which said lamp of low impedance is energized to be of a particular polarity whenever the silicon controlled rectifier associated with the one winding of said commutating transformer is triggered, thereby inducing a voltage in the other winding of said commutating transformer, through which said lamp of high impedance has been energized, of a polarity to cause disabling of the other silicon controlled rectifier. 